Enterotoxicity of El Tor-like hemolysin of non-O1 Vibrio cholerae

Modulation of Host-Microbe Metabolism by Cholera Toxin

In order for successful fecal-oral transmission, enteric bacterial pathogens have to successfully compete with the intestinal microbiota and reach high concentrations during infection. Vibrio cholerae requires cholera toxin (CT) to cause diarrheal disease, which is thought to promote the fecal-oral transmission of the pathogen. Besides inducing diarrheal disease, the catalytic activity of CT also alters host intestinal metabolism, which promotes the growth of V. cholerae during infection through the acquisition of host-derived nutrients. Furthermore, recent studies have found that CT-induced disease activates a niche-specific suite of V. cholerae genes during infection, some of which may be important for fecal-oral transmission of the pathogen. Our group is currently exploring the concept that CT-induced disease promotes the fecal-oral transmission of V. cholerae by modulating both host and pathogen metabolism. Furthermore, the role of the intestinal microbiota in pathogen growth and transmission during toxin-induced disease merits further investigation. These studies open the door to investigating whether other bacterial toxins also enhance pathogen growth and transmission during infection, which may shed light on the design of novel therapeutics for intervention or prevention of diarrheal diseases.

Pore formation-independent cell death induced by a β-barrel pore-forming toxin

Vibrio cholerae cytolysin (VCC) is a β-barrel pore-forming toxin (β-PFT). It exhibits potent hemolytic activity against erythrocytes that appears to be a direct outcome of its pore-forming functionality. However, VCC-mediated cell-killing mechanism is more complicated in the case of nucleated mammalian cells. It induces apoptosis in the target nucleated cells, mechanistic details of which are still unclear. Furthermore, it has never been explored whether the ability of VCC to trigger programmed cell death is stringently dependent on its pore-forming activity. Here, we show that VCC can evoke hallmark features of the caspase-dependent apoptotic cell death even in the absence of the pore-forming ability. Our study demonstrates that VCC mutants with abortive pore-forming hemolytic activity can trigger apoptotic cell death responses and cytotoxicity, similar to those elicited by the wild-type toxin. VCC as well as its pore formation-deficient mutants display prominent propensity to translocate to the target cell mitochondria and cause mitochondrial membrane damage. Therefore, our results for the first time reveal that VCC, despite being an archetypical β-PFT, can kill target nucleated cells independent of its pore-forming functionality. These findings are intriguing for a β-PFT, whose destination is generally expected to remain limited on the target cell membranes, and whose mode of action is commonly attributed to the membrane-damaging pore-forming ability. Taken together, our study provides critical new insights regarding distinct implications of the two important virulence functionalities of VCC for the V. cholerae pathogenesis process: hemolytic activity for iron acquisition and cytotoxicity for tissue damage by the bacteria.

Glu289 residue in the pore-forming motif of Vibrio cholerae cytolysin is important for efficient β-barrel pore formation

Vibrio cholerae cytolysin (VCC) is a potent membrane-damaging β-barrel pore-forming toxin (β-PFT). Upon binding to the target membranes, VCC monomers first assemble into oligomeric pre-pore intermediates, and subsequently transform into transmembrane β-barrel pores. VCC harbors a designated pore-forming motif, which, during oligomeric pore formation, inserts into the membrane and generates a transmembrane β-barrel scaffold. It remains an enigma how the molecular architecture of the pore-forming motif regulates the VCC pore-formation mechanism. Here, we show that a specific pore-forming motif residue, E289, plays crucial regulatory roles in the pore-formation mechanism of VCC. We find that the mutation of E289A drastically compromises pore-forming activity, without affecting the structural integrity and membrane-binding potential of the toxin monomers. Although our single-particle cryo-EM analysis reveals wild type-like oligomeric β-barrel pore formation by E289A-VCC in the membrane, we demonstrate that the mutant shows severely delayed kinetics in terms of pore-forming ability that can be rescued with elevated temperature conditions. We find that the pore-formation efficacy of E289A-VCC appears to be more profoundly dependent on temperature as compared to that of the wild type toxin. Our results suggest that the E289A mutation traps membrane-bound toxin molecules in the pre-pore-like intermediate state that is hindered from converting into the functional β-barrel pores by a large energy barrier, thus highlighting the importance of this residue for the pore-formation mechanism of VCC.

Type VI secretion system-associated FHA domain protein TagH regulates the hemolytic activity and virulence of Vibrio cholerae

The type VI secretion system (T6SS) and hemolysin HlyA are important virulence factors in Vibrio cholerae. The forkhead-associated (FHA) domain is a conserved phosphopeptide binding domain that exists in many regulatory modules. The FHA domain protein-encoding gene is conserved in the T6SS gene cluster and regulates the assembly and secretion of the T6SS. This study shows for the first time that the FHA domain protein TagH plays a role in controlling the hemolytic activity of V. cholerae, in addition to regulating the T6SS. TagH negatively regulates HlyA expression at the transcriptional and post-translational levels. The phosphopeptide binding sites of the FHA domain of TagH play a key role in the regulation of hemolytic activity. The deletion of tagH enhances the intestinal pathogenicity and extraintestinal invasion ability of V. cholerae, which mainly depend on the expression of HlyA. This study provides evidence that helps unravel the novel regulatory role of TagH in HlyA and provides critical insights which will aid in the development of strategies to manage HlyA.

Pufferfish (Tetraodon cutcutia) Sampled from a Freshwater River Serves as an Intermediate Reservoir of a Sucrose Nonfermenting Variant of Vibrio cholerae PS-4

We describe the genomic characteristics of Vibrio cholerae strain PS-4 that is unable to ferment sucrose on a thiosulfate citrate bile salt sucrose (TCBS) agar medium. This bacterium was isolated from the skin mucus of a freshwater pufferfish. The genome of strain PS-4 was sequenced to understand the sucrose nonfermenting phenotype. The gene encoding the sucrose-specific phosphotransferase system IIB (sucR) was absent, resulting in the defective sucrose fermenting phenotype. In contrast, genes encoding the glucose-specific transport system IIB (ptsG) and fructose-specific transport system IIB (fruA) showed acid production while growing with respective sugars. The overall genome relatedness indices (OGRI), such as in silico DNA-DNA hybridization (isDDH), average nucleotide identity (ANI), and average amino acid identity (AAI), were above the threshold value, that is, 70% and 95 to 96%, respectively. Phylogenomic analysis based on genome-wide core genes and the nonrecombinant core genes showed that strain PS-4 clustered with Vibrio cholerae ATCC 14035T. Further, genes encoding cholera toxin (ctx), zonula occludens toxin (zot), accessory cholera enterotoxin (ace), toxin-coregulated pilus (tcp), and lipopolysaccharide biosynthesis (rfb) were absent. PS-4 showed hemolytic activity and reacted strongly to the R antibody. Therefore, the Vibrio cholerae from the pufferfish adds a new ecological niche of this bacterium. IMPORTANCE Vibrio cholerae is native of aquatic environments. In general, V. cholerae ferments sucrose on thiosulfate citrate bile salt sucrose (TCBS) agar and produces yellow colonies. V. cholerae strain PS-4 described in this study is a sucrose nonfermenting variant associated with pufferfish skin and does not produce yellow colonies on TCBS agar. Genes encoding sucrose-specific phosphotransferase system IIB (sucR) were absent. The observed phenotype in the distinct metabolic pathway indicates niche-specific adaptive evolution for this bacterium. Our study suggests that the nonfermenting phenotype of V. cholerae strains on TCBS agar may not always be considered for species delineation.

Classic and exertional heatstroke

In the past two decades, record-breaking heatwaves have caused an increasing number of heat-related deaths, including heatstroke, globally. Heatstroke is a heat illness characterized by the rapid rise of core body temperature above 40 °C and central nervous system dysfunction. It is categorized as classic when it results from passive exposure to extreme environmental heat and as exertional when it develops during strenuous exercise. Classic heatstroke occurs in epidemic form and contributes to 9-37% of heat-related fatalities during heatwaves. Exertional heatstroke sporadically affects predominantly young and healthy individuals. Under intensive care, mortality reaches 26.5% and 63.2% in exertional and classic heatstroke, respectively. Pathological studies disclose endothelial cell injury, inflammation, widespread thrombosis and bleeding in most organs. Survivors of heatstroke may experience long-term neurological and cardiovascular complications with a persistent risk of death. No specific therapy other than rapid cooling is available. Physiological and morphological factors contribute to the susceptibility to heatstroke. Future research should identify genetic factors that further describe individual heat illness risk and form the basis of precision-based public health response. Prioritizing research towards fundamental mechanism and diagnostic biomarker discovery is crucial for the design of specific management approaches.

Curcumin Inhibits Membrane-Damaging Pore-Forming Function of the β-Barrel Pore-Forming Toxin Vibrio cholerae Cytolysin

Vibrio cholerae cytolysin (VCC) is a β-barrel pore-forming toxin (β-PFT). Upon encountering the target cells, VCC forms heptameric β-barrel pores and permeabilizes the cell membranes. Structure-function mechanisms of VCC have been extensively studied in the past. However, the existence of any natural inhibitor for VCC has not been reported yet. In the present study, we show that curcumin can compromise the membrane-damaging activity of VCC. Curcumin is known to modulate a wide variety of biological processes and functions. However, the application of curcumin in the physiological scenario often gets limited due to its extremely poor solubility in the aqueous environment. Interestingly, we find that VCC can associate with the insoluble fraction of curcumin in the aqueous medium and thus gets separated from the solution phase. This, in turn, reduces the availability of VCC to attack the target membranes and thus blocks the membrane-damaging action of the toxin. We also observe that the soluble aqueous extract of curcumin, generated by the heat treatment, compromises the pore-forming activity of VCC. Interestingly, in the presence of such soluble extract of curcumin, VCC binds to the target membranes and forms the oligomeric assembly. However, such oligomers appear to be non-functional, devoid of the pore-forming activity. The ability of curcumin to bind to VCC and neutralize its membrane-damaging activity suggests that curcumin has the potential to act as an inhibitor of this potent bacterial β-PFT.

Virulence of Cholera Toxin Gene-Positive Vibrio cholerae Non-O1/non-O139 Strains Isolated From Environmental Water in Kolkata, India

Cholera toxin (CT)-producing Vibrio cholerae O1 and O139 cause acute diarrheal disease and are proven etiological agents of cholera epidemics and pandemics. On the other hand, V. cholerae non-O1/non-O139 are designated as non-agglutinable (NAG) vibrios and are not associated with epidemic cholera. The majority of NAG vibrios do not possess the gene for CT (ctx). In this study, we isolated three NAG strains (strains No. 1, 2, and 3) with ctx from pond water in Kolkata, India, and examined their pathogenic properties. The enterotoxicity of the three NAG strains in vivo was examined using the rabbit ileal intestinal loop test. Strain No. 1 induced the accumulation of fluid in the loop, and the volume of fluid was reduced by simultaneous administration of anti-CT antiserum into the loop. The volume of fluid in the loop caused by strains No. 2 and 3 was small and undetectable, respectively. Then, we cultured these three strains in liquid medium in vitro at two temperatures, 25°C and 37°C, and examined the amount of CT accumulated in the culture supernatant. CT was accumulated in the culture supernatant of strain No.1 when the strain was cultured at 25°C, but that was low when cultured at 37°C. The CT amount accumulated in the culture supernatants of the No. 2 and No. 3 strains was extremely low at both temperature under culture conditions examined. In order to clarify the virulence properties of these strains, genome sequences of the three strains were analyzed. The analysis showed that there was no noticeable difference among three isolates both in the genes for virulence factors and regulatory genes of ctx. However, vibrio seventh pandemic island-II (VSP-II) was retained in strain No. 1, but not in strains No. 2 or 3. Furthermore, it was revealed that the genotype of the B subunit of CT in strain No. 1 was type 1 and those of strains No. 2 and 3 were type 8. Histopathological examination showed the disappearance of villi in intestinal tissue exposed to strain No. 1. In addition, fluid accumulated in the loop due to the action of strain No. 1 had hemolytic activity. This indicated that strain No. 1 may possesses virulence factors to induce severe syndrome when the strain infects humans, and that some strains of NAG vibrio inhabiting pond water in Kolkata have already acquired virulence, which can cause illness in humans. There is a possibility that these virulent NAG vibrios, which have acquired genes encoding factors involved in virulence of V. cholerae O1, may emerge in various parts of the world and cause epidemics in the future.

Impact of a reduced water salinity on the composition of Vibrio spp. in recirculating aquaculture systems for Pacific white shrimp (Litopenaeus vannamei) and its possible risks for shrimp health and food safety

Tropical shrimp, like Litopenaeus vannamei, in land-based recirculating aquaculture systems (RAS) are often kept at low water salinities to reduce costs for artificial sea salt and the amount of salty wastewater. Although these shrimp are tolerant against low salinities, innate immunity suppression and changes in the microbial composition in the water can occur. As especially Vibrio spp. are relevant for shrimp health, alterations in the species composition of the Vibrio community were analysed in water from six RAS, run at 15‰ or 30‰. Additionally, pathogenicity factors including pirA/B, VPI, toxR, toxS, vhh, vfh, tdh, trh, flagellin genes and T6SS1/2 of V. parahaemolyticus were analysed. The Vibrio composition differed significantly depending on water salinity. In RAS at 15‰, higher numbers of the potentially pathogenic species V. parahaemolyticus, V. owensii and V. campbellii were detected, and especially in V. parahaemolyticus, various pathogenicity factors were present. A reduced salinity may therefore pose a higher risk of disease outbreaks in shrimp RAS. Because some of the detected pathogenicity factors are relevant for human health, this might also affect food safety. In order to produce healthy shrimp as a safe food for human consumption, maintaining high water salinities seems to be recommendable.

Tyrosine in the hinge region of the pore-forming motif regulates oligomeric β-barrel pore formation by Vibrio cholerae cytolysin

β-barrel pore-forming toxins perforate cell membranes by forming oligomeric β-barrel pores. The most crucial step is the membrane-insertion of the pore-forming motifs that create the transmembrane β-barrel scaffold. Molecular mechanism that regulates structural reorganization of these pore-forming motifs during β-barrel pore-formation still remains elusive. Using Vibrio cholerae cytolysin as an archetypical example of the β-barrel pore-forming toxin, we show that a key tyrosine residue (Y321) in the hinge region of the pore-forming motif plays crucial role in this process. Mutation of Y321 abrogates oligomerization of the membrane-bound toxin protomers, and blocks subsequent steps of pore-formation. Our study suggests that the presence of Y321 in the hinge region of the pore-forming motif is crucial for the toxin molecule to sense membrane-binding, and to trigger essential structural rearrangements required for the subsequent oligomerization and pore-formation process. Such a regulatory mechanism of pore-formation by V. cholerae cytolysin has not been documented earlier in the structurally related β-barrel pore-forming toxins.

Interbacterial competition and anti-predatory behaviour of environmental Vibrio cholerae strains

Vibrio cholerae isolates responsible for cholera pandemics represent only a small portion of the diverse strains belonging to this species. Indeed, most V. cholerae are encountered in aquatic environments. To better understand the emergence of pandemic lineages, it is crucial to discern what differentiates pandemic strains from their environmental relatives. Here, we studied the interaction of environmental V. cholerae with eukaryotic predators or competing bacteria and tested the contributions of the haemolysin and the type VI secretion system (T6SS) to those interactions. Both of these molecular weapons are constitutively active in environmental isolates but subject to tight regulation in the pandemic clade. We showed that several environmental isolates resist amoebal grazing and that this anti‐grazing defense relies on the strains' T6SS and its actincross‐linking domain (ACD)‐containing tip protein. Strains lacking the ACD were unable to defend themselves against grazing amoebae but maintained high levels of T6SS‐dependent interbacterial killing. We explored the latter phenotype through whole‐genome sequencing of 14 isolates, which unveiled a wide array of novel T6SS effector and (orphan) immunity proteins. By combining these in silico predictions with experimental validations, we showed that highly similar but non‐identical immunity proteins were insufficient to provide cross‐immunity among those wild strains.

Characterization of Vibrio cholerae isolates from freshwater sources in northwest Ohio

Vibrio cholerae is a natural inhabitant of aquatic ecosystems worldwide, typically residing in coastal or brackish water. While more than 200 serogroups have been identified, only serogroups O1 and O139 have been associated with epidemic cholera. However, infections other than cholera can be caused by nonepidemic, non-O1/non-O139 V. cholerae strains, including gastroenteritis and extraintestinal infections. While V. cholerae can also survive in freshwater, that is typically only observed in regions of the world where cholera is endemic. We recently isolated V. cholerae from several locations in lakes and rivers in northwest Ohio. These isolates were all found to be non-O1/non-O139 V. cholerae strains, that would not cause cholera. However, these isolates contained a variety of virulence genes, including ctxA, rtxA, rtxC, hlyA, and ompU. Therefore, it is possible that some of these isolates have the potential to cause gastroenteritis or other infections in humans. We also investigated the relative motility of the isolates and their ability to form biofilms as this is important for V. cholerae survival in the environment. We identified one isolate that forms very robust biofilms, up to 4x that of our laboratory strains. Finally, we investigated the susceptibility of these isolates to a panel of antibiotics. We found that many of the isolates showed decreased susceptibility to some of the antibiotics tested, which could be of concern. While we do not know if these isolates are pathogenic to humans, increased surveillance to better understand the public health risk to the local community should be considered.

Spatiotemporal Regulation of Vibrio Exotoxins by HlyU and Other Transcriptional Regulators

After invading a host, bacterial pathogens secrete diverse protein toxins to disrupt host defense systems. To ensure successful infection, however, pathogens must precisely regulate the expression of those exotoxins because uncontrolled toxin production squanders energy. Furthermore, inappropriate toxin secretion can trigger host immune responses that are detrimental to the invading pathogens. Therefore, bacterial pathogens use diverse transcriptional regulators to accurately regulate multiple exotoxin genes based on spatiotemporal conditions. This review covers three major exotoxins in pathogenic Vibrio species and their transcriptional regulation systems. When Vibrio encounters a host, genes encoding cytolysin/hemolysin, multifunctional-autoprocessing repeats-in-toxin (MARTX) toxin, and secreted phospholipases are coordinately regulated by the transcriptional regulator HlyU. At the same time, however, they are distinctly controlled by a variety of other transcriptional regulators. How this coordinated but distinct regulation of exotoxins makes Vibrio species successful pathogens? In addition, anti-virulence strategies that target the coordinating master regulator HlyU and related future research directions are discussed.

Nonhemolysis of epidemic El Tor biotype strains of Vibrio cholerae is related to multiple functional deficiencies of hemolysin A

Background

Hemolysis of bacteria is an important phenotype used for typing and characterizing strains with specific biomarkers and even a virulence factor in bacterial pathogenesis. In Vibrio cholerae, hemolysin HlyA is responsible for hemolysis of sheep red blood cells, and this hemolytic phenotype is used as a biotyping indicator and considered one of the virulence factors. At the beginning of the seventh cholera pandemic, the El Tor biotype strains of serogroup O1 were distinguished by hemolysis from the sixth pandemic O1 classical biotype strains, whereas during the following epidemics, nonhemolytic El Tor strains appeared, suggesting phenotypic and genetic variations in these strains. This study aimed to investigate the possible mechanisms involved in nonhemolysis of El Tor strains.

Results

Five sequence types of hlyA genes were found in the studied O1 El Tor strains isolated during the seventh pandemic. A 4-base deletion in hlyA caused the HlyA protein mutation and non-hemolytic phenotype. Some strains carry wildtype hlyA genes but are still non-hemolytic, and greatly reduced hlyA transcription and blocked secretion of hemolysin were observed in hemolysis tests of the subcellular components and transcription/expression analysis of hlyA.

Conclusions

Mechanisms responsible for nonhemolysis of the epidemic O1 El Tor strains are complex and not only confined to gene mutation but also deficiencies of transcription and extracellular transport of HlyA. Mutations in gene regulation and protein secretion systems of HlyA in the nonhemolytic V. cholerae strains should be areas of concern in future studies.

Molecular insights into Vibrio cholerae's intra-amoebal host-pathogen interactions

Vibrio cholerae, which causes the diarrheal disease cholera, is a species of bacteria commonly found in aquatic habitats. Within such environments, the bacterium must defend itself against predatory protozoan grazers. Amoebae are prominent grazers, with Acanthamoeba castellanii being one of the best-studied aquatic amoebae. We previously showed that V. cholerae resists digestion by A. castellanii and establishes a replication niche within the host's osmoregulatory organelle. In this study, we decipher the molecular mechanisms involved in the maintenance of V. cholerae's intra-amoebal replication niche and its ultimate escape from the succumbed host. We demonstrate that minor virulence features important for disease in mammals, such as extracellular enzymes and flagellum-based motility, have a key role in the replication and transmission of V. cholerae in its aqueous environment. This work, therefore, describes new mechanisms that provide the pathogen with a fitness advantage in its primary habitat, which may have contributed to the emergence of these minor virulence factors in the species V. cholerae.

Expression of Hemolysin Is Regulated Under the Collective Actions of HapR, Fur, and HlyU in Vibrio cholerae El Tor Serogroup O1

The biotype El Tor of serogroup O1 and most of the non-O1/non-O139 strains of Vibrio cholerae can produce an extracellular pore-forming toxin known as cholera hemolysin (HlyA). Expression of HlyA has been previously reported to be regulated by the quorum sensing (QS) and the regulatory proteins HlyU and Fur, but lacks the direct evidence for their binding to the promoter of hlyA. In the present work, we showed that the QS regulator HapR, along with Fur and HlyU, regulates the transcription of hlyA in V. cholerae El Tor biotype. At the late mid-logarithmic growth phase, HapR binds to the three promoters of fur, hlyU, and hlyA to repress their transcription. At the early mid-logarithmic growth phase, Fur binds to the promoters of hlyU and hlyA to repress their transcription; meanwhile, HlyU binds to the promoter of hlyA to activate its transcription, but it manifests direct inhibition of its own gene. The highest transcriptional level of hlyA occurs at an OD₆₀₀ value of around 0.6–0.7, which may be due to the subtle regulation of HapR, Fur, and HlyU. The complex regulation of HapR, Fur, and HlyU on hlyA would be beneficial to the invasion and pathogenesis of V. cholerae during the different infection stages.

Plasma and Mucosal Immunoglobulin M, Immunoglobulin A, and Immunoglobulin G Responses to the Vibrio cholerae O1 Protein Immunome in Adults With Cholera in Bangladesh

Background.

Cholera is a severe dehydrating illness of humans caused by toxigenic strains of Vibrio cholerae O1 or O139. Identification of immunogenic V. cholerae antigens could lead to a better understanding of protective immunity in human cholera.

Methods.

We probed microarrays containing 3652 V. cholerae antigens with plasma and antibody-in-lymphocyte supernatant (ALS, a surrogate marker of mucosal immune responses) from patients with severe cholera caused by V. cholerae O1 in Bangladesh and age-, sex-, and ABO-matched Bangladeshi controls. We validated a subset of identified antigens using enzyme-linked immunosorbent assay.

Results.

Overall, we identified 608 immunoreactive V. cholerae antigens in our screening, 59 of which had higher immunoreactivity in convalescent compared with acute-stage or healthy control samples (34 in plasma, 39 in mucosal ALS; 13 in both sample sets). Identified antigens included cholera toxin B and A subunits, V. cholerae O–specific polysaccharide and lipopolysaccharide, toxin coregulated pilus A, sialidase, hemolysin A, flagellins (FlaB, FlaC, and FlaD), phosphoenolpyruvate-protein phosphotransferase, and diaminobutyrate–2-oxoglutarate aminotransferase.

Conclusions.

This study is the first antibody profiling of the mucosal and systemic antibody responses to the nearly complete V. cholerae O1 protein immunome; it has identified antigens that may aid in the development of an improved cholera vaccine.

Effect of Dietary Minerals on Virulence Attributes of Vibrio cholerae

Vibrio cholerae is a water-borne pathogen responsible for causing a toxin-mediated profuse diarrhea in humans, leading to severe dehydration and death in unattended patients. With increasing reports of antibiotic resistance in V. cholerae, there is a need for alternate interventional strategies for controlling cholera. A potential new strategy for treating infectious diseases involves targeting bacterial virulence rather than growth, where a pathogen’s specific mechanisms critical for causing infection in hosts are inhibited. Since bacterial motility, intestinal colonization and cholera toxin are critical components in V. cholerae pathogenesis, attenuating these virulence factors could potentially control cholera in humans. In this study, the efficacy of sub-inhibitory concentration (SIC, highest concentration not inhibiting bacterial growth) of essential minerals, zinc (Zn), selenium (Se), and manganese (Mn) in reducing V. cholerae motility and adhesion to intestinal epithelial cells (Caco-2), cholera toxin production, and toxin binding to the ganglioside receptor (GM1) was investigated. Additionally, V. cholerae attachment and toxin production in an ex vivo mouse intestine model was determined. Further, the effect of Zn, Se and Mn on V. cholerae virulence genes, ctxAB (toxin production), fliA (motility), tcpA (intestinal colonization), and toxR (master regulon) was determined using real-time quantitative PCR. All three minerals significantly reduced V. cholerae motility, adhesion to Caco-2 cells, and cholera toxin production in vitro, and decreased adhesion and toxin production in mouse intestine ex vivo (P < 0.05). In addition, Zn, Se, and Mn down-regulated the transcription of virulence genes, ctxAB, fliA, and toxR. Results suggest that Zn, Se, and Mn could be potentially used to reduce V. cholerae virulence. However, in vivo studies in an animal model are necessary to validate these results.

Anticancer and antibacterial secondary metabolites from the endophytic fungus Penicillium sp. CAM64 against multi-drug resistant Gram-negative bacteria

BACKGROUND

The emergence of multiple-drug resistance bacteria has become a major threat and thus calls for an urgent need to search for new effective and safe anti-bacterial agents.

OBJECTIVES

This study aims to evaluate the anticancer and antibacterial activities of secondary metabolites from Penicillium sp., an endophytic fungus associated with leaves of Garcinia nobilis.

METHODS

The culture filtrate from the fermentation of Penicillium sp. was extracted and analyzed by liquid chromatography-mass spectrometry, and the major metabolites were isolated and identified by spectroscopic analyses and by comparison with published data. The antibacterial activity of the compounds was assessed by broth microdilution method while the anticancer activity was determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay.

RESULTS

The fractionation of the crude extract afforded penialidin A-C (1-3), citromycetin (4), p-hydroxyphenylglyoxalaldoxime (5) and brefelfin A (6). All of the compounds tested here showed antibacterial activity (MIC = 0.50 - 128 µg/mL) against Gramnegative multi-drug resistance bacteria, Vibrio cholerae (causative agent of dreadful disease cholera) and Shigella flexneri (causative agent of shigellosis), as well as the significant anticancer activity (LC₅₀ = 0.88 - 9.21 µg/mL) against HeLa cells.

CONCLUSION

The results obtained indicate that compounds 1-6 showed good antibacterial and anticancer activities with no toxicity to human red blood cells and normal Vero cells.

Vibrio cholerae hemagglutinin(HA)/protease: An extracellular metalloprotease with multiple pathogenic activities

Vibrio cholerae of serogroup O1 and O139, the etiological agent of the diarrheal disease cholera, expresses the extracellular Zn-dependent metalloprotease hemagglutinin (HA)/protease also reported as vibriolysin. This enzyme is also produced by non-O1/O139 (non-cholera) strains that cause mild, sporadic illness (i.e. gastroenteritis, wound or ear infections). Orthologs of HA/protease are present in other members of the Vibrionaceae family pathogenic to humans and fish. HA/protease belongs to the M4 neutral peptidase family and displays significant amino acid sequence homology to Pseudomonas aeruginosa elastase (LasB) and Bacillus thermoproteolyticus thermolysin. It exhibits a broad range of potentially pathogenic activities in cell culture and animal models. These activities range from the covalent modification of other toxins, the degradation of the protective mucus barrier and disruption of intestinal tight junctions. Here we review (i) the structure and regulation of HA/protease expression, (ii) its interaction with other toxins and the intestinal mucosa and (iii) discuss the possible role(s) of HA/protease in the pathogenesis of cholera.

Signaling beyond Punching Holes: Modulation of Cellular Responses by Vibrio cholerae Cytolysin

Pore-forming toxins (PFTs) are a distinct class of membrane-damaging cytolytic proteins that contribute significantly towards the virulence processes employed by various pathogenic bacteria. Vibrio cholerae cytolysin (VCC) is a prominent member of the beta-barrel PFT (beta-PFT) family. It is secreted by most of the pathogenic strains of the intestinal pathogen V. cholerae. Owing to its potent membrane-damaging cell-killing activity, VCC is believed to play critical roles in V. cholerae pathogenesis, particularly in those strains that lack the cholera toxin. Large numbers of studies have explored the mechanistic basis of the cell-killing activity of VCC. Consistent with the beta-PFT mode of action, VCC has been shown to act on the target cells by forming transmembrane oligomeric beta-barrel pores, thereby leading to permeabilization of the target cell membranes. Apart from the pore-formation-induced direct cell-killing action, VCC exhibits the potential to initiate a plethora of signal transduction pathways that may lead to apoptosis, or may act to enhance the cell survival/activation responses, depending on the type of target cells. In this review, we will present a concise view of our current understanding regarding the multiple aspects of these cellular responses, and their underlying signaling mechanisms, evoked by VCC.

Physicochemical constraints of elevated pH affect efficient membrane interaction and arrest an abortive membrane-bound oligomeric intermediate of the beta-barrel pore-forming toxin Vibrio cholerae cytolysin

Vibrio cholerae cytolysin (VCC) is a potent membrane-damaging cytotoxic protein. VCC causes permeabilization of the target cell membranes by forming transmembrane oligomeric beta-barrel pores. Membrane pore formation by VCC involves following key steps: (i) membrane binding, (ii) formation of a pre-pore oligomeric intermediate, (iii) membrane insertion of the pore-forming motifs, and (iv) formation of the functional transmembrane pore. Membrane binding, oligomerization, and subsequent pore-formation process of VCC appear to be facilitated by multiple regulatory mechanisms that are only partly understood. Here, we have explored the role(s) of the physicochemical constraints, specifically imposed by the elevated pH conditions, on the membrane pore-formation mechanism of VCC. Elevated pH abrogates efficient interaction of VCC with the target membranes, and blocks its pore-forming activity. Under the elevated pH conditions, membrane-bound fractions of VCC remain trapped in the form of abortive oligomeric species that fail to generate the functional transmembrane pores. Such an abortive oligomeric assembly appears to represent a distinct, more advanced intermediate state than the pre-pore state. The present study offers critical insights regarding the implications of the physicochemical constraints for regulating the efficient membrane interaction and pore formation by VCC.

Transmembrane oligomeric form of Vibrio cholerae cytolysin triggers TLR2/TLR6–dependent proinflammatory responses in monocytes and macrophages

We show that the transmembrane oligomeric form of VCC evokes potent proinflammatory responses in the monocytes and macrophages of the innate immune system. VCC oligomer-induced proinflammatory responses depend critically on the TLR2/TLR6-dependent signalling pathways.

Identification of the target DNA sequence and characterization of DNA binding features of HlyU, and suggestion of a redox switch for hlyA expression in the human pathogen Vibrio cholerae from in silico studies

HlyU, a transcriptional regulator common in many Vibrio species, activates the hemolysin gene hlyA in Vibrio cholerae, the rtxA1 operon in Vibrio vulnificus and the genes of plp-vah1 and rtxACHBDE gene clusters in Vibrio anguillarum. The protein is also proposed to be a potential global virulence regulator for V. cholerae and V. vulnificus. Mechanisms of gene control by HlyU in V. vulnificus and V. anguillarum are reported. However, detailed elucidation of the interaction of HlyU in V. cholerae with its target DNA at the molecular level is not available. Here we report a 17-bp imperfect palindrome sequence, 5′-TAATTCAGACTAAATTA-3′, 173 bp upstream of hlyA promoter, as the binding site of HlyU. This winged helix-turn-helix protein binds necessarily as a dimer with the recognition helices contacting the major grooves and the β-sheet wings, the minor grooves. Such interactions enhance hlyA promoter activity in vivo. Mutations affecting dimerization as well as those in the DNA–protein interface hamper DNA binding and transcription regulation. Molecular dynamic simulations show hydrogen bonding patterns involving residues at the mutation sites and confirmed their importance in DNA binding. On binding to HlyU, DNA deviates by ∼68º from linearity. Dynamics also suggest a possible redox control in HlyU.

Nontoxigenic Vibrio cholerae non-O1/O139 isolate from a case of human gastroenteritis in the U.S. Gulf Coast

An occurrence of Vibrio cholerae non-O1/O139 gastroenteritis in the U.S. Gulf Coast is reported here. Genomic analysis revealed that the isolate lacked known virulence factors associated with the clinical outcome of a V. cholerae infection but did contain putative genomic islands and other accessory virulence factors. Many of these factors are widespread among environmental strains of V. cholerae, suggesting that there might be additional virulence factors in non-O1/O139 V. cholerae yet to be determined. Phylogenetic analysis revealed that the isolate belonged to a phyletic lineage of environmental V. cholerae isolates associated with sporadic cases of gastroenteritis in the Western Hemisphere, suggesting a need to monitor non-O1/O139 V. cholerae in the interest of public health.

Outer membrane vesicles mediate transport of biologically active Vibrio cholerae cytolysin (VCC) from V. cholerae strains

Background

Outer membrane vesicles (OMVs) released from Gram-negative bacteria can serve as vehicles for the translocation of virulence factors. Vibrio cholerae produce OMVs but their putative role in translocation of effectors involved in pathogenesis has not been well elucidated. The V. cholerae cytolysin (VCC), is a pore-forming toxin that lyses target eukaryotic cells by forming transmembrane oligomeric β-barrel channels. It is considered a potent toxin that contributes to V. cholerae pathogenesis. The mechanisms involved in the secretion and delivery of the VCC have not been extensively studied.

Methodology/Principal Findings

OMVs from V. cholerae strains were isolated and purified using a differential centrifugation procedure and Optiprep centrifugation. The ultrastructure and the contents of OMVs were examined under the electron microscope and by immunoblot analyses respectively. We demonstrated that VCC from V. cholerae strain V:5/04 was secreted in association with OMVs and the release of VCC via OMVs is a common feature among V. cholerae strains. The biological activity of OMV-associated VCC was investigated using contact hemolytic assay and epithelial cell cytotoxicity test. It showed toxic activity on both red blood cells and epithelial cells. Our results indicate that the OMVs architecture might play a role in stability of VCC and thereby can enhance its biological activities in comparison with the free secreted VCC. Furthermore, we tested the role of OMV-associated VCC in host cell autophagy signalling using confocal microscopy and immunoblot analysis. We observed that OMV-associated VCC triggered an autophagy response in the target cell and our findings demonstrated for the first time that autophagy may operate as a cellular defence mechanism against an OMV-associated bacterial virulence factor.

Conclusion/Significance

Biological assays of OMVs from the V. cholerae strain V:5/04 demonstrated that OMV-associated VCC is indeed biologically active and induces toxicity on mammalian cells and furthermore can induce autophagy.

Trapping of Vibrio cholerae cytolysin in the membrane-bound monomeric state blocks membrane insertion and functional pore formation by the toxin

Vibrio cholerae cytolysin (VCC) is a potent membrane-damaging cytolytic toxin that belongs to the family of β barrel pore-forming protein toxins. VCC induces lysis of its target eukaryotic cells by forming transmembrane oligomeric β barrel pores. The mechanism of membrane pore formation by VCC follows the overall scheme of the archetypical β barrel pore-forming protein toxin mode of action, in which the water-soluble monomeric form of the toxin first binds to the target cell membrane, then assembles into a prepore oligomeric intermediate, and finally converts into the functional transmembrane oligomeric β barrel pore. However, there exists a vast knowledge gap in our understanding regarding the intricate details of the membrane pore formation process employed by VCC. In particular, the membrane oligomerization and membrane insertion steps of the process have only been described to a limited extent. In this study, we determined the key residues in VCC that are critical to trigger membrane oligomerization of the toxin. Alteration of such key residues traps the toxin in its membrane-bound monomeric state and abrogates subsequent oligomerization, membrane insertion, and functional transmembrane pore-formation events. The results obtained from our study also suggest that the membrane insertion of VCC depends critically on the oligomerization process and that it cannot be initiated in the membrane-bound monomeric form of the toxin. In sum, our study, for the first time, dissects membrane binding from the subsequent oligomerization and membrane insertion steps and, thus, defines the exact sequence of events in the membrane pore formation process by VCC.

The β-prism lectin domain of Vibrio cholerae hemolysin promotes self-assembly of the β-pore-forming toxin by a carbohydrate-independent mechanism

Vibrio cholerae cytolysin/hemolysin (VCC) is an amphipathic 65-kDa β-pore-forming toxin with a C-terminal β-prism lectin domain. Because deletion or point mutation of the lectin domain seriously compromises hemolytic activity, it is thought that carbohydrate-dependent interactions play a critical role in membrane targeting of VCC. To delineate the contributions of the cytolysin and lectin domains in pore formation, we used wild-type VCC, 50-kDa VCC (VCC(50)) without the lectin domain, and mutant VCC(D617A) with no carbohydrate-binding activity. VCC and its two variants with no carbohydrate-binding activity moved to the erythrocyte stroma with apparent association constants on the order of 10(7) M(-1). However, loss of the lectin domain severely reduced the efficiency of self-association of the VCC monomer with the β-barrel heptamer in the synthetic lipid bilayer from ∼83 to 27%. Notably, inactivation of the carbohydrate-binding activity by the D617A mutation marginally reduced oligomerization to ∼77%. Oligomerization of VCC(50) was temperature-insensitive; by contrast, VCC self-assembly increased with increasing temperature, suggesting that the process is driven by entropy and opposed by enthalpy. Asialofetuin, the β1-galactosyl-terminated glycoprotein inhibitor of VCC-induced hemolysis, promoted oligomerization of 65-kDa VCC to a species that resembled the membrane-inserted heptamer in stoichiometry and morphology but had reduced global amphipathicity. In conclusion, we propose (i) that the β-prism lectin domain facilitated toxin assembly by producing entropy during relocation in the heptamer and (ii) that glycoconjugates inhibited VCC by promoting its assembly to a water-soluble, less amphipathic oligomer variant with reduced ability to penetrate the bilayer.

Cryo-electron microscopy reveals the membrane insertion mechanism of V. cholerae hemolysin

Vibrio cholerae hemolysin (HlyA) is a 65 kDa pore-forming toxin which causes lysis of target eukaryotic cells by forming heptameric channels in the plasma membrane. Deletion of the 15 kDa C-terminus β-prism carbohydrate-binding domain generates a 50 kDa truncated variant (HlyA50) with 1000-fold-reduced pore-forming activity. Previously, we showed by cryo-electron microscopy that the two toxin oligomers have central channels, but the 65 kDa toxin oligomer is a seven-fold symmetric structure with bowl-, ring-, and arm-like domains, whereas the 50 kDa oligomer is an asymmetric jar-like heptamer. In the present study, we determined three-dimensional(3D) structures of HlyA and HlyA50 in presence of erythrocyte stroma and observed that interaction of the 65 kDa toxin with the stroma induced a significant decrease in the height of the β-barrel oligomer with a change in conformation of the ring- and arm-like domains of HlyA. These features were absent in interaction of HlyA50 with stroma. We propose that this conformational transition is critical for membrane-insertion of the toxin.

Synergistic effect of various virulence factors leading to high toxicity of environmental V. cholerae non-O1/ non-O139 isolates lacking ctx gene : comparative study with clinical strains

Background

Vibrio cholerae non-O1/ non-O139 serogroups have been reported to cause sporadic diarrhoea in humans. Cholera toxins have been mostly implicated for hypersecretion of ions and water into the small intestine. Though most of the V. cholerae non-O1/ non-O139 strains lack these cholera toxins, several other innate virulence factors contribute towards their pathogenicity. The environmental isolates may thus act as reservoirs for potential spreading of these virulence genes in the natural environment which may cause the emergence of epidemic-causing organisms.

Results

The environmental isolates of vibrios were obtained from water samples, zooplanktons and phytoplanktons, from a village pond in Gandhinagar, Gujarat, India. They were confirmed as Vibrio cholerae non-O1/ non-O139 using standard biochemical and serotyping tests. PCR experiments revealed that the isolates lacked ctxA, ctxB, tcpA, zot and ace genes whereas other pathogenicity genes like toxR, rtxC, hlyA, hapA and prtV were detected in these isolates. Compared with epidemic strain V. cholerae O1 El Tor N16961, culture supernatants from most of these isolates caused higher cytotoxicity to HT29 cells and higher hemolytic, hemagglutinin and protease activities. In rabbit ileal loop assays, the environmental isolates showed only 2-4 folds lesser fluid accumulation in comparison to N16961 and a V. cholerae clinical isolate IDH02365 of 2009. Pulsed Field Gel electrophoresis and Random amplification of Polymorphic DNA indicated that these isolates showed considerable diversity and did not share the same clonal lineage even though they were derived from the same water source. All the isolates showed resistance to one or more antibiotics.

Conclusion

Though these environmental isolates lacked the cholera toxins, they seem to have adopted other survival strategies by optimally utilising a diverse array of several other toxins. The current findings indicate the possibility that these isolates could cause some gastroenteric inflammation when ingested and may serve as progenitors for overt disease-causing organisms.

Fatal diarrheal disease caused by Vibrio cholerae O67 in a patient with myelodysplastic syndrome

A 71-year-old man with myelodysplastic syndrome (MDS) receiving treatment with azacitidine developed extensive watery diarrhea for three consecutive days. As a result of high-grade dehydration, the patient was urgently admitted to the hospital and fluid replacement therapy was initiated. However, the patient's diarrhea did not improve. Vibrio cholerae non-O1/non-O139 was detected in a fecal culture. On the fourth day, the patient died due to circulatory collapse. An autopsy revealed extensive necrosis of the intestinal mucosa. Vibrio cholerae non-O1/non-O139-induced diarrheal disease often develops in patients with hepatic cirrhosis and has a serious clinical course. We herein report a fatal outcome of Vibrio cholerae O67 infection in an immunocompromised MDS patient.

Functional mapping of the lectin activity site on the β-prism domain of vibrio cholerae cytolysin: implications for the membrane pore-formation mechanism of the toxin

Vibrio cholerae cytolysin (VCC) is a prominent member in the family of β-barrel pore-forming toxins. It induces lysis of target eukaryotic cells by forming transmembrane oligomeric β-barrel channels. VCC also exhibits prominent lectin-like activity in interacting with β1-galactosyl-terminated glycoconjugates. Apart from the cytolysin domain, VCC harbors two lectin-like domains: the β-Trefoil and the β-Prism domains; however, precise contribution of these domains in the lectin property of VCC is not known. Also, role(s) of these lectin-like domains in the mode of action of VCC remain obscure. In the present study, we show that the β-Prism domain of VCC acts as the structural scaffold to determine the lectin activity of the protein toward β1-galactosyl-terminated glycoconjugates. Toward exploring the physiological implication of the β-Prism domain, we demonstrate that the presence of the β-Prism domain-mediated lectin activity is crucial for an efficient interaction of the toxin toward the target cells. Our results also suggest that such lectin activity may act to regulate the oligomerization ability of the membrane-bound VCC toxin. Based on the data presented here, and also consistent with the existing structural information, we propose a novel mechanism of regulation imposed by the β-Prism domain's lectin activity, implicated in the process of membrane pore formation by VCC.

Genomic analysis of immune response against Vibrio cholerae hemolysin in Caenorhabditis elegans

Vibrio cholerae cytolysin (VCC) is among the accessory V. cholerae virulence factors that may contribute to disease pathogenesis in humans. VCC, encoded by hlyA gene, belongs to the most common class of bacterial toxins, known as pore-forming toxins (PFTs). V. cholerae infects and kills Caenorhabditis elegans via cholerae toxin independent manner. VCC is required for the lethality, growth retardation and intestinal cell vacuolation during the infection. However, little is known about the host gene expression responses against VCC. To address this question we performed a microarray study in C. elegans exposed to V. cholerae strains with intact and deleted hlyA genes.

Many of the VCC regulated genes identified, including C-type lectins, Prion-like (glutamine [Q]/asparagine [N]-rich)-domain containing genes, genes regulated by insulin/IGF-1-mediated signaling (IIS) pathway, were previously reported as mediators of innate immune response against other bacteria in C. elegans. Protective function of the subset of the genes up-regulated by VCC was confirmed using RNAi. By means of a machine learning algorithm called FastMEDUSA, we identified several putative VCC induced immune regulatory transcriptional factors and transcription factor binding motifs. Our results suggest that VCC is a major virulence factor, which induces a wide variety of immune response- related genes during V. cholerae infection in C. elegans.

Hemolysin induces Toll-like receptor (TLR)-independent apoptosis and multiple TLR-associated parallel activation of macrophages

Vibrio cholerae hemolysin (HlyA) displays bipartite property while supervising macrophages (MΦ). The pore-forming toxin causes profound apoptosis within 3 h of exposure and in parallel supports activation of the defying MΦ. HlyA-induced apoptosis of MΦ remains steady for 24 h, is Toll-like receptor (TLR)-independent, and is driven by caspase-9 and caspase-7, thus involving the mitochondrial or intrinsic pathway. Cell activation is carried forward by time dependent up-regulation of varying TLRs. The promiscuous TLR association of HlyA prompted investigation, which revealed the β-prism lectin domain of HlyA simulated TLR4 up-regulation by jacalin, a plant lectin homologue besides expressing CD86 and type I cytokines TNF-α and IL-12. However, HlyA cytolytic protein domain up-regulated TLR2, which controlled CD40 for continuity of cell activation. Expression of TOLLIP before TLR2 and TLR6 abrogated TLR4, CD40, and CD86. We show that the transient expression of TOLLIP leading to curbing of activation-associated capabilities is a plausible feedback mechanism of MΦ to deploy TLR2 and prolong activation involving CD40 to encounter the HlyA cytolysin domain.

The role of C-terminus carbohydrate-binding domain of Vibrio cholerae haemolysin/cytolysin in the conversion of the pre-pore β-barrel oligomer to a functional diffusion channel

BACKGROUND & OBJECTIVES

Vibrio cholerae cytolysin/hemolysin (VCC) is a 65 kDa pore-forming toxin (PFT) secreted by O1 El Tor and non-O1 strains. The purified toxin, which contains two C-terminus carbohydrate-binding domains in addition to the cytolytic domain at the core, causes lysis of a wide spectrum of eukaryotic cells at picomolar concentrations, apoptogenesis of intestinal and immune cells and accumulation of fluid in rabbit ligated ileal loop. Therefore, it may potentially complement the action of cholera toxin (CT) in diarrheagenic strains that do not produce CT. We showed earlier that β1-galactosyl-terminated glycoconjugates are strong inhibitors of its pore-forming activity, though carbohydrates are not functional receptors of VCC. Here, we investigate how the 15 kDa C-terminus β-prism lectin domain contributed to pore formation in erthrocytes.

METHODS

VCC was isolated from the culture supernatant of late log phase grown bacteria and purified to homogeneity by chromatography. The 50 kDa truncated variant was generated by restricted proteolysis. Liposome was prepared by sonication of a suspension of phospholipids and calceine release assay was done by spectrofluorometric monitoring of the released dye trapped in liposome. Formation of β-barrel oligomers in erythrocyte stroma was monitored by scanning electron microscopy.

RESULTS

Proteolytic truncation of the C-terminus β-prism lectin domain decreased hemolytic activity of the toxin by ~800-fold without causing a significant change in pore-forming activity toward synthetic lipid vesicles devoid of incorporated glycoproteins/glycolipids. Truncation at the C-terminus did not impair membrane-binding or assembly to the oligomeric pore.

INTERPRETATION & CONCLUSIONS

Our data indicated that the C-terminus domain played a critical role in translocation of the pre-pore oligomeric assembly from the cell surface or lipid-water interface to the hydrocarbon core of the membrane bilayer, signaling the formation of functional diffusion channels.

Vibrio cholerae hemolysin is required for lethality, developmental delay, and intestinal vacuolation in Caenorhabditis elegans

BACKGROUND

Cholera toxin (CT) and toxin-co-regulated pili (TCP) are the major virulence factors of Vibrio cholerae O1 and O139 strains that contribute to the pathogenesis of disease during devastating cholera pandemics. However, CT and TCP negative V. cholerae strains are still able to cause severe diarrheal disease in humans through mechanisms that are not well understood.

METHODOLOGY/PRINCIPAL FINDINGS

To determine the role of other virulence factors in V. cholerae pathogenesis, we used a CT and TCP independent infection model in the nematode Caenorhabditis elegans and identified the hemolysin A (hlyA) gene as a factor responsible for animal death and developmental delay. We demonstrated a correlation between the severity of infection in the nematode and the level of hemolytic activity in the V. cholerae biotypes. At the cellular level, V. cholerae infection induces formation of vacuoles in the intestinal cells in a hlyA dependent manner, consistent with the previous in vitro observations.

CONCLUSIONS/SIGNIFICANCE

Our data strongly suggest that HlyA is a virulence factor in C. elegans infection leading to lethality and developmental delay presumably through intestinal cytopathic changes.

vttRA and vttRB Encode ToxR family proteins that mediate bile-induced expression of type three secretion system genes in a non-O1/non-O139 Vibrio cholerae strain

Strain AM-19226 is a pathogenic non-O1/non-O139 serogroup Vibrio cholerae strain that does not encode the toxin-coregulated pilus or cholera toxin but instead causes disease using a type three secretion system (T3SS). Two genes within the T3SS pathogenicity island, herein named vttR(A) (locus tag A33_1664) and vttR(B) (locus tag A33_1675), are predicted to encode proteins that show similarity to the transcriptional regulator ToxR, which is found in all strains of V. cholerae. Strains with a deletion of vttR(A) or vttR(B) showed attenuated colonization in vivo, indicating that the T3SS-encoded regulatory proteins play a role in virulence. lacZ transcriptional reporter fusions to intergenic regions upstream of genes encoding the T3SS structural components identified growth in the presence of bile as a condition that modulates gene expression. Under this condition, VttR(A) and VttR(B) were necessary for maximal gene expression. In contrast, growth in bile did not substantially alter the expression of a reporter fusion to the vopF gene, which encodes an effector protein. Increased vttR(B) reporter fusion activity was observed in a DeltavttR(B) strain background, suggesting that VttR(B) may regulate its own expression. The collective results are consistent with the hypothesis that T3SS-encoded regulatory proteins are essential for pathogenesis and control the expression of selected T3SS genes.

Quorum sensing negatively regulates hemolysin transcriptionally and posttranslationally in Vibrio cholerae

Recent work has shown that in addition to cholera toxin (CT) and the toxin-coregulated pilus (TCP), other cytotoxic proteins in Vibrio cholerae also cause disease symptoms, and this is particularly evident in strains lacking CT. One such protein is the hemolysin encoded by hlyA. Here we show that, like CT and TCP, HlyA is repressed by the quorum-sensing-regulated transcription factor HapR. This repression occurs on two levels: one at the transcriptional level that is independent of the metalloprotease HapA and one at the posttranslational level that is mediated by HapA. The transcriptional regulation is significantly more apparent on solid media than in liquid cultures. This is the first time that hemolysis has been shown to be directly regulated by quorum sensing in V. cholerae, and it is interesting that, like other virulence factors, HlyA is also repressed by HapR, which is expressed late in infection.

The Vibrio cholerae cytolysin promotes chloride secretion from intact human intestinal mucosa

Background

The pathogenicity of the Vibrio cholerae strains belonging to serogroup O1 and O139 is due to the production of virulence factors such as cholera toxin (CT) and the toxin-coregulated pilus (TCP). The remaining serogroups, which mostly lack CT and TCP, are more frequently isolated from aquatic environmental sources than from clinical samples; nevertheless, these strains have been reported to cause human disease, such as sporadic outbreaks of watery diarrhoea and inflammatory enterocolitis. This evidence suggested the possibility that other virulence factor(s) than cholera toxin might be crucial in the pathogenesis of Vibrio cholerae-induced diarrhoea, but their nature remains unknown. VCC, the hemolysin produced by virtually all Vibrio cholerae strains, has been proposed as a possible candidate, though a clear-cut demonstration attesting VCC as crucial in the pathogenesis of Vibrio cholerae-induced diarrhoea is still lacking.

Methodology/Principal Findings

Electrophysiological parameters and paracellular permeability of stripped human healthy colon tissues, obtained at subtotal colectomy, mounted in Ussing chamber were studied in the presence or absence of VCC purified from culture supernatants of V. cholerae O1 El Tor strain. Short circuit current (I_SC) and transepithelial resistance (R_T) were measured by a computerized voltage clamp system. The exposure of sigmoid colon specimens to 1 nM VCC resulted in an increase of I_SC by 20.7%, with respect to the basal values, while R_T was reduced by 12.3%. Moreover, increase in I_SC was abolished by bilateral Cl⁻ reduction.

Conclusion/Significance

Our results demonstrate that VCC, by forming anion channels on the apical membrane of enterocytes, triggers an outward transcellular flux of chloride. Such an ion movement, associated with the outward movement of Na⁺ and water, might be responsible for the diarrhoea caused by the non-toxigenic strains of Vibrio cholerae.